The present invention relates to a method for machining pistons incorporated in variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a method for forming shoe receiving recesses in pistons.
A typical compressor includes a crank chamber defined in a housing. A drive shaft is rotatably supported in the housing. Part of the housing is constituted by a cylinder block. The cylinder block includes cylinder bores, each of which accommodates a piston. A swash plate is fitted to the drive shaft in the crank chamber and supported to rotate integrally with the drive shaft. Each piston is coupled with the peripheral portion of the swash plate by shoes. The swash plate converts the rotation of the drive shaft to linear reciprocation of the pistons. The reciprocation of the pistons compresses refrigerant gas.
Each piston has a pair of concave recesses facing each other for slidably receiving a pair of shoes. Japanese Unexamined Patent Publication No. 6-249140 describes a method for forming such recesses in pistons. The method will be described with reference to FIG. 6.
As shown in FIG. 6, a slot is formed in a skirt 41a of a piston 41. The slot includes walls 42A and 42B, which face each other. A ball end mill 43 is placed between the walls 42A and 42B such that the axis L2 of the end mill 43 is perpendicular to the axis L1 of the piston 41. The ball end mill 43 includes a shank 43a and a spherical cutter 43b fixed to the distal end of the shank 43a. The center C2 of the cutter 43b is located at a midpoint (an initial position P1) between the walls 42A and 43B and on the axis L1 of the piston 41.
The ball end mill 43 is rotated about its axis L2. The piston 41 is then moved along its axis L1 in one direction by such an amount that the center C2 of the cutter 43b is moved with respect to the piston 41 from the initial position P1 to a first machining position P2. This causes the cutter 43b to form a concave recess 44A in the wall 42A. Subsequently, the piston 41 is moved along its axis L1 in the other direction by such an amount that the center C2 of the cutter 43b is moved from the first machining position P2 to a second machining position P3. This causes the cutter to form another concave recess 44B in the wall 42B.
In the method of FIG. 6, the recess 44A defines part of a first sphere, the center of which is a first machining position P2, and the recess 44B defines part of a second sphere, the center of which is the second machining position P3. As shown in FIG. 6, the center P2 is displaced from the center P3 by a distance S4.
However, ideally, the centers of the spheres should coincide to smoothly convert rotation of the drive shaft into linear reciprocation of the pistons via the swash plate. In other words, the recesses 44A and 44B should preferably conform to a single sphere. A variable displacement compressor includes a swash plate that is tiltably supported on a drive shaft. The inclination of the swash plate is changed for varying the displacement of the compressor. In such a compressor, the swash plate must be smoothly tilted. For optimally smooth tilting of the swash plate, it is important to accurately machine the recesses 44A, 44B so that the recesses 44A, 44B define a single sphere.
However, the method of FIG. 6 forms the recesses 44A and 44B so that they conform to different spheres, which hinders the performance of the resulting compressor.